A typical design constraint, for marine craft other than certain specialized racing or record breaking boats, is that, unlike the wings of an aircraft, thrust wings must work as well with the wind coming from one side, or tack, as when it comes from the other side. In an aircraft, that is equivalent to the design of a plane capable of exactly the same performance upside down as right way up. In the specialized example of planes specifically designed for aerobatics, this is indeed sometimes the case.
This means that a thrust wing aerofoil section for marine use must be symmetrical about a vertical plane, or that it must be constructed from more or less vertical aerofoil components, each of which is symmetrical about its own vertical plane of symmetry, and each of which is hinged to another at the intersections of each plane of symmetry, so that any special arrangement of the components to increase thrust, or reduce drag, or both, on one tack may be recreated in a mirror image for sailing on the opposite tack.
Known examples include the wingsails fitted to the ‘Zefyr’ series of wingsail yachts. In this design a symmetrical wing is provided with a symmetrical external flap hinged to the wing at the intersection of their planes of symmetry. A third element, a symmetrical air directing slat, is also hinged to the wing at the intersection of their planes of symmetry. Careful selection of hinge axes and of the appropriate deflection angles of flap and slat can produce a deeply asymmetrical arrangement capable of extremely high levels of thrust at well-contained levels of drag, and also capable of being turned into its mirror image for sailing on the opposite tack.
However, the known wingsail has a relatively high level of complexity. For example, the flap is usually power operated by an electric or hydraulic actuator, and in some configurations a further flap locking system has been incorporated.